During the normal course of use for many systems, a source of power will be removed and reconnected over time. Each time the power is reconnected, there may be an opportunity to connect the power improperly. For example, in battery powered applications, a battery may be inserted backwards. In rechargeable systems, a battery charger may be connected incorrectly, or a non-compatible battery charger may be connected. In other systems, a power supply component may be connected to the system incorrectly. A reverse battery, battery charger or power supply connection is dangerous because parasitic diodes of the internal circuits and even ESD (Electronic Static Discharge) circuits can be forward biased and draw a large current. These large currents may damage the ESD structures and internal circuits. Circuits that limit the reverse current in order to protect the on-chip circuits as well as ESD circuits are typically located off-chip as shown in FIG. 1.
Integrated circuit (IC) 100 includes functional circuitry 102 that is protected by ESD circuits at each pin, such as ESD circuit 104. An off-chip circuit 112 protects the IC when a high reverse voltage is inadvertently connected to supply voltage pin (VDD) 110. In this example, the protection circuit is a simple resister. The higher the resistance of resistor R, the better the protection effect will be since reverse current is limited to a lower value by the resistor. However, when the impedance of R is large, the normal current that the normal connection can provide is limited. For example, assume R=10K, VDD=40V normally or −40V if reversely connected. The ESD protected pin 106 is clamped to about −0.6V when reverse connected. This value may vary depending on the structure and process parameters for IC 100. The reverse current is −0.6V−(−40)V/10 k=3.94 mA. In normal condition, when VDD=40V, voltage in input pin 106 would also be limited by resistor R based on current; 40V−10K*I. In order to keep the internal power level above 2V, for example, then Imax would be 3.8 mA. This is not good for many applications.
Another way is to place a diode shown as FIG. 2A in protection circuit 212. The voltage drop from VDD to the voltage supply pin 106 is determined by the diode forward voltage. The forward impedance is small. But the voltage drop of the diode becomes a larger portion of lower values of VDD. For example, when VDD=2V, diode voltage drop=0.6V, internal power supply voltage at pin 106 is limited to 1.4V, assuming the current load is not too high and the diode area is reasonably large. The above method requires the diode to survive high reverse bias voltage.
A better way may be to place a diode in parallel to the resistor shown as FIG. 2B in protection circuit 222. The voltage drop from VDD to the voltage supply pin 106 is determined by the R value, the diode area, diode forward clamp voltage and the current load. The forward impedance is smaller than that in FIG. 1. But there is still a limitation of the voltage drop due to the diode. As VDD is reduced, the diode voltage drop of around 0.6V becomes a larger portion of VDD. For example, VDD=2V, voltage drop=0.6V, internal power supply voltage at pin 106 is limited to 1.4V, assuming the current load is not too high and the diode area is reasonably large. The above method also requires the diode to survive high reverse bias voltage.